1. Field of the Invention
The subject invention generally pertains to a system for detecting the presence of a body near a door and more specifically to a system that includes an antenna for sensing the body.
2. Description of Related Art
There are a wide variety of available devices for detecting the presence of a body, such as a person or object, near a doorway. Such detection devices, known as proximity sensors, photoelectric eyes, motion detectors, etc., operate under various principles including, ultrasonics; active and passive detection of infrared radiation; detection of electromagnetic radiation (including sensing radio waves or sensing changes in capacitance or impedance); detecting a Doppler shift in microwaves; and lasers. In response to sensing a nearby body, the detector may simply trigger a light or an alarm, or the device may affect the operation of a door.
Some proximity sensors comprise an antenna that creates an electromagnetic field along the leading edge of a vertically operating door. When a nearby body disturbs the field, the sensor may trigger a controller to stop or reverse the closing action of the door. If the antenna moves vertically with the door and the controller and power supply are stationary, then wiring between the antenna and the controller must allow for the movement of the antenna. This can be accomplished by using a flexible coiled cable between the controller and the antenna. A coiled cable, however, has its limitations.
Many doors, for instance, have a breakaway feature that allows a door to temporarily break away from its vertical guide tracks should a collision occur between the door and a vehicle or other obstruction. During the collision, the breakaway feature allows the door to yield without permanently damaging the door or its guide tracks. After the collision, the door is readily restored to its normal operation. Although a coiled cable may have sufficient flexibility to allow a door to open and close, such a cable may get entangled with the door, track or vehicle during a collision.
Thus, a need exists for a way to use an antenna-based proximity sensor on a door that has a breakaway feature.